Magnetic tunnel junctions and spin valves are commonly used as MRAM storage bits and magnetic read heads. The discovery that the magnetization of a free layer can be manipulated by means of a transferred spin polarized current, enables the magnetization of the free layer of such a structure to be manipulated in such a way that the magnetization of the layer keeps rotating. The resulting oscillation in resistance of the structure can be utilized for switching of MRAM elements or as an oscillating component, a spin torque oscillator (STO).
A spin valve is typically a sandwiched structure consisting of two ferromagnetic layers separated by a thin non-magnetic spacer layer. A magnetic tunnel junction (MTJ) would have the same structure but the non-magnetic layer would in this case be insulating, allowing for tunneling between the two ferromagnetic layers.
One of the ferromagnetic layers is said to be the “pinned layer” because it is magnetically pinned or oriented in a fixed direction. This can be achieved by applying an adjacent anti ferromagnetic layer, a pinning layer, which will affect and hold the magnetization of the pinned layer through exchange coupling. The other ferromagnetic layer is called the free layer since its magnetization is allowed to rotate in response to outside stimulus in form of magnetic fields or as in this case action by spin torque (transferred magnetic angular momentum from electrons).
The total resistance of the layered structure will depend on the orientation of the fixed and pinned layers. A parallel configuration will give a lower resistance than the anti parallel case.
The spin torque effect is when polarized electrons moving into the free layer of the structure transfer a net magnetic moment to the layer, offsetting the Gilbert damping of the magnetization. This can make the free layer magnetization switch direction or cause the magnetization to oscillate, depending on the amount of magnetic moment being transferred, that is, the amount of current per unit area passing in to the layer.
A spin valve or magnetic tunnel junction with a free layer performing such an oscillator is called a spin torque oscillator.
It has been shown experimentally that spin torque oscillators can phase lock to each other forming an oscillating signal of higher quality than the individual signals.
A spin torque oscillator will have many practical benefits. The lateral size is very small, typically below 100 nm, a dimension significantly smaller than the classical oscillators based on LC tanks or dielectric resonance. When integrated in a microchip an STO will save space and cost in different applications. Furthermore the frequency generated in an STO is sweepable by means of the current passed trough the device. Additionally, the frequency can be adjusted by applying different external fields.
Applications range from mobile phones, satellite communication, radar and ship transceivers.
At the present time the output signal of a single STO is too weak for practical applications. J. Grollier et al (Grollier, J.; Cros, V. & Fert, A. Synchronization of spin-transfer oscillators driven by stimulated microwave currents, Phys. Rev. B, 2006, 73, 060409-0604012) has theoretically proposed that STOs may be connected in series in order to increase the power of the output signal. The STOs will phase lock under some specific circumstances and will in the phase locked state provide a signal less sensitive to noise and with higher output.
However, discrepancies in fabrication mean that the shape-anisotropy fields of magnetic layers in the spin torque oscillators can differ greatly, especially as elements get smaller. Also, edge effects and roughness has a profound effect on the anisotropy of small magnetic structures. According to experimental results, a shape anisotropy field difference of 4% is the maximum tolerance for broad-band phase-locking behavior.
When the size of components reach the nano level, surface roughness becomes an increasing problem, and edge effects automatically introduce a variation in the effective anisotropy field. This will make an effective phase lock through serial connection hard to achieve.